1. Field of the Invention
The present invention relates to a semiconductor storage device and a semiconductor storage device driving method.
2. Related Art
Nonvolatile semiconductor storage devices such as NAND flash memories are frequently used in portable devices such as digital cameras and cellular phones. Such a semiconductor storage device is required to be low in power dissipation, low in current dissipation, and fast in operation.
Such a semiconductor storage device includes a memory cell array formed on a well in a semiconductor substrate. The memory cell array is divided into a large number of blocks, and each block includes a plurality of memory cells. One well is formed so as to extend over a plurality of blocks. Typically, one well is provided so as to extend over the entire memory cell. When erasing data in memory cells included in one block, therefore, it is necessary to charge a large-capacity well provided over a plurality of blocks. Hereafter, a well potential needed to erase data in memory cells is referred to as erasing potential.
The well is boosted by using booster circuits of a plurality of kinds sometimes. For example, the well is boosted by booster circuits of two kinds: a booster circuit BCpgm and a booster circuit BCpass. The booster circuit BCpgm supplies a write voltage to memory cells into which data is to be written, i.e., selected cells. The booster circuit BCpass supplies a write inhibition voltage to memory cells into which data is not to be written, i.e., inhibited memory cells. The booster circuit BCpgm has more booster stages than the booster circuit BCpass. As a result, the booster circuit BCpgm can conduct boosting to a higher potential than the booster circuit BCpass. On the other hand, the booster circuit BCpass is less in power dissipation than the booster circuit BCpgm.
In order to boost the well to an erasing potential at the time of data erasing, either a method using only the booster circuit BCpgm (method 1) or a method using both the booster circuits BCpgm and BCpass (method 2) is conventionally used.
In the method 1, it takes a considerably long time to boost the large-capacity well to the erasing potential. Therefore, it is difficult for the method 1 to meet the demand of fast operation of semiconductor storage devices which will become more intensive hereafter. In the method 2, boosting is conducted by using both booster circuits BCpgm and BCpass, and then boosting is conducted by using only the booster circuit BCpgm. In the method 2, therefore, the well can be boosted in a short time. In the method 2, however, both the booster circuits BCpgm and BCpass are used, resulting in a problem that the instantaneous peak of the current dissipation becomes too large.
In both the method 1 and the method 2, the booster circuit BCpgm is mainly used to boost the well. Since the booster circuit BCpgm has more booster stages than BCpass, the booster circuit BCpgm is poor in boosting efficiency. Therefore, these methods also have a problem that considerably much power is dissipated to boost the well.
Therefore, a semiconductor storage device and a semiconductor storage device driving method that are low in power dissipation, low in current dissipation, and fast in operation speed are provided.